High power two-dimensional diode laser arrays dissipate large amounts of power over a small spatial extent, and are simultaneously subject to stringent constraints on operating temperature. Efficient cooling of the lasers in such arrays is vital to their normal operation.
In a commonly used cooling design approach, the lasers are brought in thermal contact with a heat sink through corresponding heat-conductive submounts (mounting modules). For given heat sink and laser temperatures, the heat flow through a submount depends on the submount geometry and on the thermal properties of the submount materials. In particular, the heat flow through a module decreases with the distance between its corresponding laser and the heat sink.
At the same time, it is important that individual lasers (or monolithic linear laser arrays, also called "bars") be tested individually, or burned in, before the assembly of the two-dimensional array. Individual burning-in ensures that a defect in one array element does not require the replacement of an entire array. Burning-in an element of the 2-D array is typically done by thermally connecting the module to a test heat sink, electrically connecting the N and P sides of the bar to test leads, and running a current through the bar.
Since laser bars are very fragile, it is important to ensure that the bars are protected from mechanical stresses during testing. Such mechanical stresses arise, for example, from pressing a test lead directly on a bar. To address the testing problem stemming from the fragility of bars, a conventional array element includes a wire bond plate mounted on the module, between the bar and the base plate, for providing an electrical connection to the surface of the bar opposite the module. For more information on conventional packaging architectures for diode laser arrays, see for example U.S. Pat. No. 4,716,568. Wire leads are connected from the surface of the bar to the test leads. During testing, the test leads are pressed on the module and the wire bond plate. Thus, the laser bar is not placed in potentially-damaging mechanical contact with the test leads.
The wire bond plate also serves, after stacking, to provide an electrical communication between its corresponding bar and the module of an adjacent bar. The lasing sections of an individual bar are connected in parallel, while different bars are connected in series. The serier conductive path runs through a module, its corresponding laser, and its corresponding wire bond plate to the module of an adjacent laser.
The conventional wire-bond-plate (WBP) architecture requires relatively tall modules, and thus leads to sub-optimal heat flow through the modules. The high thermal resistance of WBP modules, coupled with the relatively poor thermal conductivity of the removable connection between the module and the test heat sink, prohibit the full-power burn-in of the bars. Burn-in at low powers, in turn, requires long periods of time. No general method has been established for simultaneously reducing the distance between the laser bars and the base plate, and allowing robust testing of the bars before assembly into an array of high-power bars connected in series.
Various designs for laser diode modules are discussed in a number of U.S. Patents. For more information on conventional WBP module designs for high-power two-dimensional diode laser arrays, see for example U.S. Pat. Nos. 4,716,568 and 5,099,488, which are herein incorporated by reference.
In U.S. Pat. No. 5,495,490, Rice et al. describe a diode laser array comprising laser bars mounted on thermally conductive plates, and connected in series. Cooling fluid flows between the plates and cools the laser bars. The array described by Rice et al. does not permit individual burning-in of its elements, however. Moreover, the array is cooled by fluid, rather than by conduction of heat to a base plate. In U.S. Pat. No. 4,454,602, Smith describes a one-dimensional array of individual diode lasers wired in series. Each diode is sandwiched between two conductive plates. The distance between the diodes and the heat sink is relatively large, and the thermal conductivity of the heat-transfer assembly is poor.